Travelling wave tubes



Jan. 27, 1959 D. c. ROGERS EI'AL 2,871,392

TRAVELLING WAVE TUBES Filed April 21, 1955 W hm Inventors D C.ROGERS' W. J. POHL By A Home y England, assignors to International Standard Electric Corporation, New York, N. Y.

Application April 21, 1955, Serial No. 502,811 Claims priority, application Great Britain June 21, 1954 3 Claims. (Cl. 3153.5)

The present invention relates to low noise travelling wave tubes.

It has previously been proposed to construct a low noise travelling wave tube wtih an electron gun having electrodes arranged to provide, in a uniform magnetic field, a beam which, after passing through a first ac- United States Patent celerating anode, is then subject to successive accelerations in each of two further regions before entering a drift tube preceding the helix of the travelling wave amplifier proper.

sothat the beam tends to diverge from the cathode towards the first anode, rather than tending to be parallel, the noise figure of the amplifier is reduced by about 2 db. As stated above, the electron gun is immersed in a substantially uniform magnetic field, parallel with the beam axis.

The present applicants have found, using an electron gun arrangement, of the kind defined above and defining, herein after the type referred to that the noise figure may be further improved by arranging that in the region between the cathode and the first anode the magnetic field diverges instead of being uniform.

According to the present invention, therefore, there is provided a low noise traveling wave amplifier of the type referred to in which the magnetic field between the cathode and the first anode is divergent whereby the noise figure is reduced.

According to a further aspect of the invention there is provided a low noise travelling wave amplifier comprising an electron gun having a cathode protruding through a low potential shield electrode, a first anode at a first accelerating potential, second and third anodes at successively higher accelerating potentials, the spacing between first and second and between second and third anodes being of the order of twice the spacing between the cathode and the first anode; a helical line and conventional arrangements for coupling the helical line to a source of input signals; a drift tube situated between the said third anode and the helical line of such length that the mean square noise velocity modulation of the electron beam is a minimum in the region where the helical line is coupled to the said source; and arrangements for providing a magnetic focusing field along the axis of the helix diverging from behind the cathode and becoming uniform along the helix.

In carrying out the invention the divergence of the magnetic field may be effected, for example, by using a series of focusing coils about the travelling wave tube, the flux of the first coil at the cathode end being greater than that of the remainder. Alternatively a pole piece may be arranged to distort the field at the cathode end of the tube.

The invention will now be described with reference to the accompanying drawings in which:

Fig. 1 shows, purely diagrammatically, the electrode arrangement of the electron gun used in the invention and Fig. 2 shows the arrangement of focusing coils and the structure in which the travelling wave tube is mounted.

In this prior construction it is found that when the beam current and gun potentials are arranged "ice tion is essentially one described in the paper Low-Noise Travelling-Wave Amplifier, by R. W. Peter published in R. C. A. Review, vol. 13, No. 3, September, 1952, at pages 344-368. The electron gun comprises a cathode, indicated at 1 in the accompanying Fig. l, protruding from its mounting 2 through the aperture in'a low potential shield electrode 3, which is of re-entrant form, the cathode emitting surface being substantially in the plane of the disc'portion of the shield. A first apertured anode 4 is spaced in front of the cathode as closed as is practical. A second anode 5, having a somewhat larger aperture, is spaced from the first anode by about twice the separation between cathode and first anode. A third anode 6, spaced from the second anode by about the same I distance, is shown integral with a drift tube 7. The travelling wave helical line 8 is connected by pick-up probe 9 to the other end of drift tube 7. The probe 9 and about one turn of the helical line lie within the amplifier input waveguide indicated by the dotted lines 10.

In Fig. l the planes of the cathode and shield electrode and of the several anodes are indicated at C, S and A A A respectively. The regions CA A -A A A may be regarded as drift spaces in which the beam electrons are subject to velocity changes somewhat analogous to those which occur in a velocity jump amplifier, the potentials and drift lengths being proportional in terms of the beam current so that the noise velocity modulation impressed on the beam at the virtual cathode gives rise to electron space charge waves in drift tube 7, the mean square noise velocity becoming a minimum at the end of the drift tube. V

In a practical embodiment the spacings between the several planes indicated in Fig. 1 and the electrode voltages are given below.

Plane Voltage Distance from cathode The drift tube 7 is 2% inches (7.28 cm.) long. For a' With a magnetic field of 325 gauss uniform over-'the whole of the electron beam to within plus or minus five percent, two microamperes of beam current were collected by the helix and drift tube and the noise figure was found to be about 10 db. Subsequently experiments were made to check the noise performance of the tube under variations fro-m the design conditions. It was found that when the unfo-rmity of the magnetic field in the electron gun region was deliberately made non-uniform, so as to be divergent, the noise figure was improved by a far greater margin than was expected. Further experiments proved that this effect was not due to other accidental causes and that an optimum improvement was obtained when the magnetic field in the C--A region diverged from cathode to anode at a rate of about 2.1 in alength of 1 inch (2.54 cm.). For the same beam current and helix interception a 2 db improvement in noise figure was obtained when the measured magnetic field on the beam axis varied in the following manner.

Distance from cathode: Field, gauss 3 inches (7.6 cm.) and beyond 320-330 It should be explained that in the prior arrangement, using a magnetic field everywhere parallel to the beam axis, were the magnetic field removed, the electron beam would fan out from the cathode in thecathode-first anode/region. In the presence of the axial magnetic fieldthe diverging electrons, having instantaneous velocities at. an angle to the lines of magnetic flux, are subject to Lorentzforces which cause them to rotate around the individual magnetic flux lines and. hence to counteract the tendency for the beam to diverge. Initially there will be some divergence and thereafter the beam Will contract somewhat. With an initially divergent magnetic field, since the angle between the instantaneous electron velocities and: themagnetic flux lines is reduced, the beam divergence canbe expected to be greater than in the axial magnetic field case. Fromtwhat has been pointed out in the above mentioned paper by Peter, itis to be expected that an. increase in the beam divergence in the cathode first anode region would have a beneficial effect on the noise figure, but, the reduction in noise figure to be expected' from the magnetic field divergence used in embodi ments of the present invention is an order of magnitude less than that actually obtained.

The ararngem'ent of the tube in its mounting structure is shown in Fig. 2. The travelling wave tube 11 is mounted from the gun end by a platform 12 to which the base 13- of, the tube is connected. At the collector end the tube is supported in an assembly end plate 14 with the collector electrode protruding through, as indicated at 15. The platform 12 is mounted on a second end plate 16'and an assembly of focussing coils, 17 to 22 inclusive, is clamped between the end plates by means of tie bars 23- of magneticmaterial. These tie bars, together with the end plates 14 and 16, form a magnetic enclosure about the focussing coils. The assembly also includes, mounted integrally therewith, input and output waveguides 24 and.25. respectively, the waveguides being apertured to receive the travelling wave tube and being provided with matching terminations 26 and 27 respectively.

At the electron gun end of the tube an inner pole piece member 28 is shown clamped to the end plate 16 and projecting within the coil 17. To obtain a uniform magnetic field when the energising current through all the focusing coils is the same we usually provide a rearward extension of the cylindrical portion of the member 28. In this embodiment this extension has been omitted and the. cathode l is situated about one inch (25 cms.) in frontv of the cylindrical portion of the pole piece. The re-entrant pole piece causes the field immediately in front of it to diverge in the manner indicated in the above table;

As an alternative arrangement, when the pole piece is omitted otherwise arranged to provide a uniform magnetic field with equal current to all the focusing coils, we have found that the required field divergence can be obtained by increasing the energising current flowing through coil .17.

What is claimed is:

1. A low noise travelling wave amplifier comprising: an electron gun having a cathode, a plurality of anodes at successively higher accelerating potentials longitudinally spaced from said cathode and. from each other, a helical line and means for coupling said helical line to a source of input signals, and means for providing a magnetic focussing field alongthe' axis of the' helical line and encompassing said electron gun diverging from behind the cathode and becoming: uniform along the helical line, said magnetic focussing means comprising a series of focussing coils spaced from said cathode along said amplifier, the flux of the coil at the cathode end being greater than.- that of the: remaining coils.

2. A low noise travelling wave amplifier according to claim 1 coinprising'a pole piece, which pole piece is positionedi in front of' said cathode to distort the magnetic field at the cathode-to make it non-uniform to provide the additional flux.

3-. A- low noise: travelling wave amplifier comprising: an; electron gun. having a cathode protruding through a low potential shield electrode, a first anode at a first acceleratingpotential: spaced longitudinally of said cathode, second and third anodes at'successively higher accelerating potentials, the longitudinally spacing between first and second and between second and third anodes being of the order of twice the spacing between the cathode and the first anode; a helical line and means for coupling said line to a source. of input signals; a drift tube situated between the said third anode and the helical line of such length that: the mean square noise velocity modulation of the electron beam is a minimum in the region wherein the;helicalline'is coupled to=the said source'yand magneticmeansfor providing a magnetic focussingfield along the axis of" thehelical line diverging frombehindthe cathoderand becoming uniform along the helical line.

References Citedin the file-'of this patent UNITED STATES- PATENTS 2,608,668' Hines Aug. 26, 1952 2,632,130 Hull -1 Mar. 17, 1953 2,741,718 Wang Apr. 10, 1956 2,792,518 Quate May 14,1957

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